Fig. 1.

An early “landscape genetics” study by Dobzhansky et al. (13) where the spatial and temporal distribution of D. pseudoobscura chromosome types was mapped across the western United States. ST, standard gene arrangement of the third chromosome. Black bars represent the frequency of the ST chromosomes in ∼1940, stippled in 1957, and white in the period 1963–1965. The dotted black rectangle is the approximate location of a suture zone later predicted by Remington (16) and confirmed by Rissler and Smith (19, 25) separating populations in the Great Plains from populations in the Pacific Coastal regions (figure without suture zone is figure 1 from ref. 13).